A typical power supply system is schematically illustrated in FIG. 1. In such a system, an AC supply such as a commercial power system 2 is typically used to supply power to a load 4 over transmission lines 6. Unfortunately, such commercial supplies fail from time to time (for example, due to lightning strikes). Such power failures are undesirable, and, in some instances, catastrophic (e.g., power to life support equipment, etc.). To avoid such problems, prior art systems have provided back-up power supplies, typically referred to as uninterruptible power supplies (UPS). Such back-up power supplies typically include an AC detection circuit 8 for monitoring the AC line voltage developed by the commercial power system 2. They also include a back-up supply 9, which usually comprises a DC power source such as a string of rechargable batteries and a DC to AC converter such as an inverter. In the event of a sufficient drop in the AC line voltage detected by the detection circuit 8, the back-up supply 9 is brought on line to supply power to the load 4.
Within this context, it is important to accurately and quickly measure the AC line voltage to identify power outages on the AC power line 6. Specifically, any delay between the occurrence of a power failure and the detection of that failure creates an interval during which power is not supplied to the load 4. Such a delay in bringing the back-up supply 9 on line can result in data loss and/or equipment failure.
In view of the foregoing, it will be appreciated that detecting power outages and providing fast and accurate information about the AC line voltages are of primary importance to UPS systems. As a result, many schemes for sensing power outages have been proposed. However, none of the proposed schemes provide accurate information about the measured AC voltage in sufficiently short time frames.
For example, in a typical prior art approach, the AC detection circuit includes one fast AC sensor for quickly identifying a power outage and one slow AC sensor to accurately measure the AC line voltage. This approach, however, is disadvantageous in that the difference in detection speed between these sensors inherently guarantees that a delay between the detection of an outage and the provision of accurate voltage information relating to that outage will always be present. Indeed, the time period between the power outage and the moment of receiving accurate information about the measured AC line voltage under such approaches is approximately one-half the time period of the AC voltage waveform on the AC power line.